The invention relates to a method and machine for fine machining shafts such as crankshafts with center bearings and eccentric bearings which are both machined in a single machining setup.
As summarized in DE 197 14 677 C2, crankshafts are conventionally often manufactured in a method whereby material was removed from the bearing locations of the originally formed, that is cast or forged, crankshaft in three subsequent machining procedures. The first machining step involved a rough cut with a geometrically determined cutting shape. Various processes were used herefor such as form cutting, milling, inside milling, outside milling etc. The material removal is in the millimeter range. The desired bearing diameters are formed with an excess diameter of a few tenths of a millimeter.
This step is generally followed by hardening procedure, in which the crankshaft is thermally tempered, rolled or otherwise treated. This step is followed by fine machining wherein particularly the main bearings and the lift bearings are ground down to the desired dimensions. During this procedure, a material amount in the area of one tenth of a millimeter is removed. Following this step, only finishing procedures are performed for generating the desired surface quality.
The above roughly described machining procedure was generally considered to have the disadvantage that it required grinding. During grinding a cooling lubricant must be applied whereby the removed material particles are wetted and forms a grinding mud. This grinding mud is problematic and must be disposed of which increases costs. With Cubic Boron Nitride grinding, known as CBN grinding, the danger of an explosion exists. In addition, large amounts of cooling lubricants are required. Still the workpieces are easily overheated.
Based hereon DE 197 14 677 C2 proposes to replace the grinding procedures by machining procedures using geometrically determined cutting edges, particularly circumferential milling or twin milling using high speed milling cutters. This procedure was to be performed without lubricating coolants (dry). During this milling procedure a surface roughness with a characteristic roughness value Ra of preferably less than two micrometers was to be generated. With this process, the support area of the bearing surface at the transition to fine machining was expected to be less than 50% and particularly less than 25% of that present after the finishing step.
Also, DE 197 49 940 A1 is concerned with elimination of the grinding step during the manufacture of crankshafts. It proposed to perform the cutting step with the geometrically determined cutting edge with such an accuracy that roundness deviation is less then ten micrometers, the diameter deviation is less than 100 micrometers in the form of positive deviation based on the desired contour after finishing and the roughness is less than two micrometers. These values are based on the requirements of the finishing process which does not allow any substantial geometry changes or dimension changes of the workpiece, but which may only affect the surface quality.
If during fine machining, a transition occurs from machining processes with a geometrically undefined cutting area that is with grinding procedures, to machining processes with geometrically determined cutting edges, that is, for example, milling procedures this in itself does not mean that the efficiency is improved. Although, even multiple cranked crankshafts of complicated multi-cylinder engines, such as, for example, V6, V8, W8, V10, V12 and W12 truck or passenger car engines are stable and rigid components, it is common practice to support the crankshaft during machining near the area which is momentarily machined, for example, by a steadying member in order to compensate for different rigidities of the camshaft in different directions or at different bearing location. The use of such-steadying members, however, is time-consuming since they must be replaced during the machining process.
As crankshaft materials are very strong steels or a casting materials such as CrMo 4 or casting iron with graphite spheres such as GGG 60, GGG 70, or GGG 80 are used because of the increasing torques to be transmitted. Occasionally, even manganese-silicon steel alloys are used. With such materials the machining sequence on the process control are very important. For example, to avoid excessive operating pressures, grinding procedures may be divided into several individual machining steps wherein each is performed with less material removal. For example, pregrinding and finish grinding is performed on different machines, the number of the revolutions in the machines is increased and/or in each machine is changed from the machining of sets to individual machining. During machine cutting with a geometrically determined cutting edge, however this is not possible. Rather, particularly during cutting of tough materials a minimum sharing thickness must be maintained whereby the workpiece is subjected to a substantial load.
In addition, cutting edges of a milling tool which periodically engage the workpiece during machining can cause the camshaft to vibrate so that bevels may be formed and which result in dimension deviations.
Although occasional attempts have been made to eliminate the grinding of crankshafts and to achieve the fine machining of crankshafts by milling procedures such procedures have not been used in large volume manufacturing because of difficulties with repeatability. Furthermore, it has strangely been found that the milling tools wear very rapidly.
It is therefore, the object of the present invention to provide means for the accurate fine machining of crankshafts with tools having a geometrically defined cutting edge.